Three journalists moderated the debate: Azucena Uresti, from Milenio, a major Mexican newspaper; Denise Maerker, from TV channel Televisa; and Sergio Sarmiento; from TV Azteca.

Key points

Ahead of the debate, Lopez Obrador, 64, led his opponents with a double-digit lead, according to opinion polls. In the televised event:

The leftist candidate was the target of criticism from his contenders.

Many of the attacks focused on a proposal by him that includes exploring the possibility of amnesty for criminals to help bring security and reconcilation to the country.

To this attack, he responded: "Amnesty does not mean impunity ... We have to attend to the original problems that led to the issue" such as combating poverty.

"Violence broke out in the country because there has been no economic growth for 30 years ... if there is no economic growth, there are no jobs, if there are no jobs, there is no well-being, and if there is no well-being there can be no peace and tranquillity."

"Mexico is a factory of poverty," he added. "Fire cannot be fought with fire."

"I propose a government that represents the poor and the rich; the government will stop serving just a minority ... I won't fail you," he added.

"Let's build a house for all, a Mexico where we all are welcome."

During the debate, Obrador pointed to a graphic, showing his with a 48 percent favourability rating in the Reforma newspaper's poll last week.

"I don't mean to brag," he said, but "something terrible has to happen" for us not to win, and we're going to win this election."

Anaya, 39, who is polling second in most surveys presented himself as the only viable option to defeat the leftist candidate. Anaya has struggled to rid himself from accusations of corruption and money laundering.

He attacked Lopez Obrador's proposals, using posters and photographs and citing specific passages of his government plan.

Anaya said he would create an independent attorney general's office to deal with political corruption. He also said the president should be investigated during its mandate, if needed, while jail should be an option for corrupted politicians.

"Mexico will change," he said "The PRI [ruling party] is leaving, the question is what kind of change do we want."

"Economically we were better, but in terms of safety, today, we are worse than ever."

Addressing the issue of tackling crime, Anaya said the current strategy had been wrongly implemented, "it has not been understood that the gangs should be dismantled, instead of just removing the leaders."

He questioned the amnesty proposed by Lopez Obrador and asked: "Andres Manuel, regarding the amnesty, does it mean forgiving the criminals, yes or no?

"There are already a million victims accumulated only in the last two presidential terms ... to forgive the delinquents is not the solution ".

During the debate, he portrayed Lopez Obrador as an old, corrupted politician. "I respected you," Anaya told him. "You are not the same any more. I don't know if it's because of your obsession with power, or you're just getting tired."

Calderon, 60, also known as "El Bronco" is the governor of Nuevo Leon. He was elected in 2015 as Mexico's first independent governor. He is in fifth place in most opinion polls.

The independent candidate caused a stir when he said "thieves should have their hands chopped off".

He added: "This works ... let's see if the Congress is brave enough to pass this law."

Calderon also said political parties are primarily to blame for the country's problems.

He suggested creating the first cyber police.

To combat corruption he said: "My first proposal is to have an independent president, that does not depend on the Congress; [also to have] an autonomous prosecutor elected by society ... we need to set the example."

Calderon also spoke about education: "A proposal to lower crime rates lies in the generation that today has no opportunities ... in Nuevo Leon all the new high schools are militarised, that is, the army is teaching ... this is giving results."

Meade, 49, who represents the ruling PRI, faced criticism for the tenure of outgoing President Enrique Pena Nieto.

Regarding insecurity, he called for better preventive and investigative policies: "Out of two million crimes each year, only three out of 100 are resolved. My wife has been robbed twice ... as long as we have corruption, there cannot be security."

He continued: "Prevention is needed, the [state] needs to have a presence, and [we need] to investigate ... if we want to be successful, we have to strengthen the investigation, so there are consequences when fighting violence.

"[During my mandate], there will be no master scams ... I propose to end impunity ... the public ministry will be autonomous.

"I propose a unique criminal code ... I am going to create a specialised investigation agency in crimes plus kidnapping and trafficking ... I will take care of the safety of your family.

"To the army, we need to give them legal certainty, we have asked a lot from our military, they deserve more security ... and the citizen deserves absolute clarity and to know what exactly we expect from them ... [we want] that space for dialogue."

Zavala, 50, is an an ex-first lady and independent candidate. She also had to respond to criticism against the decisions of her husband, former President Felipe Calderon.

Zavala advocated for strong state presence to combat corruption.

"I will defend everything that has been done well in the country ... particularly in terms of safety, one of the proposals that we have seen working is the intervention of the state," she said.

"We have to fight the evil with the good ... and that's is with the presence of the state ... because we won't have a Mexico in peace if we do not have a Mexico that feels safe," she said.

When is the next presidential debate?

The next debate will take place in Tijuana on May 20, and the last one will be held in Merida on June 12.

What were Mexicans saying online?

#DebateINE was one of the top trending topics in Mexico during the debate, these were some of the reactions.

Translation:From 2014, crime increased by 29 of 32 entities in the country. We have to change the strategy and continue to support the army until peace is restored and we have professional and reliable policemen. #DebateINE, Anaya wrote.